The present invention relates to propeller-driven fixed-wing aircraft and especially to a turbine engine drive and mounting assembly for use as either original equipment or as a conversion unit for such aircraft.
Single or twin-engine propeller-driven airplanes typically use piston engines as their prime movers. However, for many heavy-use military, agricultural, forestry, mining and other industrial applications, and for use in climate and altitude extremes and in rugged terrain with short runways, turbine engines offer significant advantages over piston engines for driving such aircraft. Turbine engines are much lighter weight than piston engines of the same horsepower. Therefore a turbine powered aircraft would have greater altitude, would require shorter takeoffs and landings, and would achieve its cruising or working altitude quicker than a comparable piston engine-powered airplane. A turbine engine is also more dependable than a piston engine, especially in very cold climates, and is easier to operate and control. It also can be operated for longer periods between overhauls and usually requires much less maintenance between overhauls than a piston engine, thereby compensating for its higher initial cost and cost of overhaul. In many cases turbine fuel is less expensive than the sometimes scarce aviation gasoline used in aircraft piston engines. Nevertheless, suitable turbo-prop gas turbine engines are not available in the configurations required for aircraft used in dirty environments and climate extremes. Therefore, fixed-wing aircraft used for such purposes continue to be driven by piston engines.
Reliable turboshaft gas turbine engines are available in the horsepower ranges reguired for propeller-driven aircraft. However, their typically high r.p.m. outputs make them unsuitable for use with aircraft unless coupled to a reduction gearbox to reduce speed to the levels required for driving a propeller. In fact, turboshaft engines coupled with gearboxes are used for driving helicopters (rotary wing aircraft) and are used in large, heavy fixed-wing planes.
In prior fixed-wing applications the turboshaft engine and gearbox have been rigidly interconnected, either by mounting the gearbox directly to the engine housing, or by suspending it rigidly from the engine housing. However, both such arrangements are undesirable for airplanes, especially light aircraft, because the high torque developed by the gearbox is transmitted directly to the engine, shortening its life and requiring unacceptably frequent major and expensive overhauls. Direct coupling of the gearbox and turboshaft engine would be especially unacceptable for light airplanes used in hostile environments where propeller strikes are a hazard, often overturning the airplane. Any shock loading or unbalanced torque loading caused by a propeller strike would be transmitted directly through the gearbox to the engine, causing severe damage and necessitating expensive repairs. Thus, with a turbine engine, which may be up to six times as expensive as a comparable piston engine, engine damage is to be avoided at all cost.
Turboshaft engines have been readily adapted for use in helicopters using gearboxes mounted entirely independently of the engines on separate mounting frames, thereby effectively isolating the engine from the high torque loads at the gearbox. However, in airplanes there are constraints of space, shape and position which do not permit independent mounting of the gearbox and engine on separate mounting frames in the same manner as helicopters.
Sand, dirt, dust, and other foreign objects are other hazards of a hostile environment which have heretofore made turboshaft engines impractical for use in light airplanes used in such environments. The compressor blades of a turboshaft engine typically rotate at up to 54,000 r.p.m., and therefore foreign objects in the huge quantities of air sucked in by the air compressors of such engines can rapidly and seriously damage such compressors. Complicating the problem is the fact that the air intakes and compressors of turboshaft engines suitable for aircraft use typically face forward toward the propeller where their exposure to foreign objects would be the greatest.
Accordingly, there is a need for a method and a means for mounting a turboshaft engine and associated drive train in fixed-wing aircraft in a manner that would protect the turbine engine from the normal high torque loads at the gearbox, the excessive torque and shock loads of a propeller strike, and foreign object damage.
A primary objective of the invention is, therefore, to provide an acceptable method and means for driving the propeller of a fixed-wing aircraft with a turboshaft engine. Another primary object is to provide a method and means as aforesaid which isolates the engine from the normal high torque loads of the gearbox and propeller-induced shock loads.
Another important object is to provide a method and means as aforesaid which protects the turbine engine from foreign object damage.
Another important object is to provide a method and means as aforesaid which also provides easy access to the engine and other drive train components for service and maintenance.
Another object is to provide a method and means as aforesaid which enables quick and inexpensive conversion of fixed-wing aircraft from a piston engine drive to a turbine engine drive.
Still another object is to provide a method and means as aforesaid which protects the engine from damage in the event of a propeller strike or an overturned aircraft.
Further objects are to provide a unitary turboshaft engine mounting and drive assembly for airplanes which is adaptable for different sizes, makes and models of such aircraft, and which can be installed in such aircraft either as original equipment or as a retrofit conversion.
Another object is to provide a turboshaft engine mounting and drive assembly for aircraft which enables the engine to provide reliable performance with low maintenance and long periods between overhauls, even when used in hostile environments.